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MOLARS OF EUTHERIA

CHAP.

of the Marsupials as ''singularly specialised characters/' in no
way intermediate in character. This view applies also to the
pouch, which, as already stated, distinguishes the adults of that
group. But the impossibility of using this last character as
one of any importance has been shown by the discovery of
rudiments of it in. embryos of undoubtedly Eutherian mammals
(see p. 18).
Less stress is laid now upon the existence of four molars in
the Marsupials as
dividing them from
the higher mammals
than was formerly the
case. The total denti-
tion of the group is
on the whole com-
posed of more numer-
ous individual teeth
than in the typical
Eutheria; but we have
FIG.   58.—Sagittal  section   of brain  of Rock   Wallaby   exceptions     like     the
(Petroyale penicillwta).     ant.com, Anterior uommis-   -rrrv...i«„       t-]\p     Arrni
sure ; cbl, cerebellum ; c.vutmt  corpus mammillare ;    vv IlcUfciSJ      ^iit/     -xirmti-
c.yu,   corpora  quadrigenmia ;   crur,   crura   cerebri ;   dillo   JPrtodonteS,   and
epi, epiphy.sis,  with the  posterior commissure im-   , ,          •..-           ,
mediately behind it ; f.nwrt, position of foramen of   *"e      IVianatee ;     or
Monro ; hip.com, luppocarupal  commissure, consist-   better,     because     free
ing here of two layers  continuous behind at  the   c             ,                 ,   .          „
spleneiura, somewhat divergent in front where the   irom   tne suspicion OI
septnm lueidum extends between them ; hypo, hypo-   Becondary  multiplica-
physis ; med,  medulla oblongata ; mid.com, middle                   J                x
commissure ; olf, olfactory lobe ; o$)t, optic chiasma ;   tion, OtOCyon and OCCa-
tient. 3, third ventricle.    (From Parker awd Haswell'ft   sionallv Taccordillff  to
Mr, Thomas) Centetes.
In the last two there are at least sometimes four molars.
On the other hand, a few archaic characters of some import-
ance crop up here and there among the Marsupials, which are
sometimes held to point to a primitive ancestry. It has been
remarked that in Marsupials it is the fourth toe which is dominant
in size, whereas in Ungulates it is the third. An attempt has
been made to explain, this on the view (reasonable enough in
itself) of a tree-living ancestry for the group, A greater develop-
ment of the fourth toe Is, however, by no means a necessary
character of arboreal creatures ; the Primates themselves are an
exception. Nor is this prevalence universal among the Marsupials;